Parkinson disease (PD) is proximally caused by degeneration of dopamine containing neurons that project from the Substantial Nigra to the striatum. These neurons form synaptic connections with structures termed dendritic spines that reside on two subpopulations of the medium spiny neuron (MSN) that express either D1 dopamine receptors or D2 dopamine receptors (D1R and D2R, respectively). The dendritic spines also receive glutamatergic synaptic inputs from the cortex that activates calcium-dependent signaling in spines. Thus, the release of dopamine differentially modulates the actions of glutamate within the dendritic spines of D1R- and D2R-containing MSNs. Dopamine-depletion in PD patients and in parkinsonian animal models results in loss of dendritic spines from striatal MSNs; however, spine loss is restricted to D2R-containing MSNs in short-term rodent studies. Moreover, emerging data from several labs indicate that corticostriatal synapses onto D1R- and D2R-containing MSNs are differentially regulated by dopamine and other neurotransmitters. However, very little is known about the biochemical differences between striatal MSN subpopulations that presumably account for these differences. This project investigates the roles of spinophilin in D1R- and D2R-containing MSNs. Spinophilin is a scaffolding protein that binds protein phosphates 1 (PP1), F-actin, and several other proteins involved in regulating cell signaling and morphology. The global knockout of spinophilin disrupts corticostriatal synaptic function in both D1R- and D2R-containing MSNs, and also affects the morphology of striatal MSNs in an age- dependent manner. My recent studies showed that dopamine depletion enhances the interaction of spinophilin with PP11; presumably modulating the dephosphorylation of other associated dendritic proteins. In order to identify potential substrates of the spinophilin-PP1 complex, I performed a proteomics screen, identifying multiple spinophilin-associated proteins (SpAPs) in normal, mature striatum that are known to regulate cell morphology, including CaMKII. Previous studies in this lab showed that dopamine depletion leads to hyper- phosphorylation of CaMKII. In this career development award I will develop innovative transgenic molecular tools to address my over-arching hypothesis that: dopamine depletion and aging differentially regulate spinophilin-dependent signaling in striatal MSN subtypes. Two aims will begin to test this hypothesis: Aim 1 will test the hypothesis that dopamine depletion alters the spinophilin interaction network in an age-dependent manner. Aim 2 will use novel transgenic animals expressing differentially tagged forms of spinophilin in D1R- or D2R- containing MSNs to test the hypothesis that dopamine depletion alters the spinophilin interaction network in a cell-specific manner. These studies will greatly enhance our understanding of spinophilin-mediated striatal signaling in animal models of PD and will inform potential drug targets for this disorder.